Almost all cells can move and motility is an important process in the normal function of many cells. Cell migration also contributes to many diseases, including cancer metastasis and atherosclerosis. Cells move in response to signals by generating lamellipodia, which extend the cell membrane, adhere and allow the cell to pull itself forward. This process does not happen without actin polymerization. Cells control actin polymerization by regulating the creation of free barbed ends on actin filaments. How signals that stimulate cell motility ultimately result in the generation of actin filaments with free barbed ends and actin polymerization is not known. The small GTP-binding protein Rac1 is a critical intermediate in many signal transduction pathways that cause cell motility. The signals stimulated by Rac1 that promote actin polymerization and lamellipodial extension and adherence are not known. Several critical steps that coordinate actin polymerization are regulated by phosphatidylinositol-4,5-bisphosphate (Ptdlns-4,5-P2). Rac1 associates with the phosphatidylinositol-4-phosphate 5-kinases (PIP5Ks), the enzymes that synthesize Ptdlns-4,5-P2. Rac also stimulates Ptdlns-4,5-P2 synthesis. This confluence of findings suggests that Rac1-stimulated synthesis of Ptdlns-4,5-P2 may play a critical role in cell motility. The goal of the current proposal is to test this hypothesis. The study of Ptdlns-4,5-P2 has been hampered by the multiple functions of Ptdlns-4,5-P2 as both a signaling molecule and a substrate. We have devised approaches that will allow us to determine the function of Ptdlns-4,5-P2 in response to Rac1 activation and determine how Rac1 regulates Ptdlns-4,5 -P2 levels. These investigations are at an important junction between signal transduction and cell biology. The results of our studies should generate significant new insight into how signal transduction pathways regulate actin polymerization. We should understand better how cells move and advance the fields of cell biology and signal transduction.